This invention pertains to Raman amplifiers and, more particularly, to Raman amplifiers utilizing a wide bandwidth continuous wave (CW) overall power spectrum pump signal.
Optical fiber technology is currently utilized in communications systems to transfer information, e.g., voice signals and data signals, over long distances as optical signals. Over such long distances, however, the strength and quality of a transmitted optical signal diminishes. Accordingly, techniques have been developed to regenerate or amplify optical signals as they propagate along an optical fiber.
One well-known amplifying technique exploits an effect called Raman scattering to amplify an incoming information-bearing optical signal (referred to herein as a xe2x80x9csignal wavelengthxe2x80x9d). Raman scattering describes the interaction of light with molecular vibrations of the material through which the light propagates (referred to herein as the xe2x80x9ctransmission mediumxe2x80x9d). Incident light scattered by molecules experiences a downshift in frequency from the power-bearing optical signal (referred to herein as the xe2x80x9cpump wavelengthxe2x80x9d). This downshift in frequency (or increase in wavelength) from the pump wavelength is referred to as the xe2x80x9cStokes Shift.xe2x80x9d The downshift of the peak gain from the pump wavelength is referred to herein as the xe2x80x9cpeak Stokes shift.xe2x80x9d The extent of the downshift and the shape of the Raman gain curve is determined by the molecular-vibrational frequency modes of the transmission medium. In amorphous materials, such as silica, molecular-vibrational frequencies spread into bands which overlap and provide a broad bandwidth gain curve. For example, in silica fibers, the gain curve extends over a bandwidth of about 300 nm from the pump wavelength and has a peak Stokes shift of about 100 nm.
The overall concept of Raman scattering is well known and is described in numerous patents and publications, for example, R. M. Stolen, E. P. Ippen, and A. R. Tynes, xe2x80x9cRaman Oscillation in Glass Optical Waveguides,xe2x80x9d Appl. Phys. Lett, 1972 v. 20, 2 PP62-64; and R. M. Stolen, E. P. Ippen, Raman Gain in Glass Optical Waveguides,xe2x80x9d Appl. Phys. Lett, 1973 v. 23, 6 pp. 276-278), both of which are incorporated herein by reference. With respect to the present invention, the most relevant aspect of Raman scattering is its effect on signal wavelengths traveling along the transmission medium.
FIG. 1 illustrates a prior art optical amplifier which utilizes Raman scattering to amplify a signal wavelength. Referring to FIG. 1, a pump wavelength xcfx89p and a signal wavelength xcfx89s are effected (e.g., co-injected) into a Raman-active transmission medium 10 (e.g., fused silicon) in opposite directions. As used herein, the term xe2x80x9ceffectedxe2x80x9d used in connection with the placement of signals on a transmission medium refers broadly to taking any action or participating in any way that results in signals being propagated onto an optical fiber. Co-propagating pumps can be used, although a counter-propagation pump scheme reduces polarization sensitivity and cross talk between wavelength division multiplexed (WDM) channels. Providing that the wavelength of the signal wavelength xcfx89s is within the Raman gain of power wavelength xcfx89p (e.g., about 300 nm in silica), the signal wavelength xcfx89s will experience optical gain generated by, and at the expense of, the pump wavelength xcfx89p. In other words, the pump wavelength xcfx89p amplifies the signal wavelength xcfx89s and, in so doing, it is diminished in strength. This gain process is called stimulated Raman scattering (SRS) and is a well-known technique for amplifying an optical signal. The two wavelengths xcfx89p and xcfx89s are referred to as being xe2x80x9cSRS coupledxe2x80x9d to each other. Isolator 16 provides unidirectional propagation and reduces multipath Rayleigh scattering in the signal bandwidth. It can also incorporate a filter which transmits all signals of the signal wavelength xcfx89s and blocks signals of the pump wavelength xcfx89p thereby filtering out the pump wavelength.
FIGS. 2A-2C illustrate the gain curve for a signal wavelength xcfx89s amplified using a single narrow band pump wavelength xcfx89p of a specific frequency. As shown in FIGS. 2B and 2C, while gain occurs over a broad bandwidth, less than about 35 nm (the area between point A and point B) is, from a practical standpoint, useable to effectively amplify the signal wavelength xcfx89s. The area between points A and B is the bandwidth where gain variation is less than 3dB, i.e., less than two times gain variation.
To increase the useable gain beyond this width, it is known to utilize multiple narrow band single wavelength pumps as described in U.S. Pat. No. 4,616,898 to Hicks, Jr., incorporated fully herein by reference. As shown in FIGS. 3A-3C, when multiple single wavelength pumps xcfx89p1, xcfx89p2, and xcfx89p3 are generated having small wavelength separations, a composite gain curve is generated from the amplified signal wavelengths xcfx89s 123 which approaches a uniform amplification level. However, as can be seen in FIG. 3C, a certain gain ripple still exists, the magnitude of which depends upon the number of signal wavelengths and the wavelength separation between them. In a pure silica fiber, the gain ripple associated with discrete pump wavelengths, although as small as 0.05 dB, is still present when the separation between the pump signals is as small as 1 nm.
Thus, while prior art multiple-pump Raman amplifiers produce an amplified output with a minimal amount of ripple in the output, it would be desirable to have a Raman amplifier producing a ripple-free output.
The present invention provides a method and apparatus by which a wide bandwidth continuous wave (CW) or substantially continuous wave composite (SCWC) pump with a substantially flat spectrum is utilized to amplify an information-carrying signal. By using a wide bandwidth CW pump, substantially no ripple is introduced to the signal being amplified by the Raman amplifier.
Accordingly, one aspect of the present invention is a method of generating a ripple-free amplified optical signal by using wide bandwidth continuous wave pumps. In a preferred embodiment, the method comprises the steps of: effecting an information-bearing optical signal onto a Raman-active transmission medium in a first direction; and effecting a wide spectral bandwidth continuous wave (CW) pump signal onto the Raman-active transmission medium in a direction opposite to that of the information-bearing optical signal. The wide spectral bandwidth CW pump signal should have a bandwidth that is greater than the bandwidth of the information-bearing optical signal, and in a more preferred embodiment, the wide spectral bandwidth CW pump signal has a bandwidth of at least 3 nm.
In an alternative embodiment, the method comprises the steps of: effecting an information-bearing optical signal onto a Raman-active transmission medium in a first direction; and effecting a wide spectral bandwidth substantially continuous wave composite (SCWC) pump signal onto the Raman-active transmission medium in a direction opposite to that of the information-bearing optical signal.
Yet another aspect of the present invention is an amplification system for generating a ripple-free amplified optical signal. In a preferred embodiment, the amplifier system comprises: a transmission medium coupled to an information-signal source; a pump source generating a wide spectral bandwidth continuous wave (CW) pump signal; and a coupler for coupling a pump signal generated by the pump source to the transmission medium. The pump source can comprise a single wide spectral bandwidth pump source or a plurality of wide spectral bandwidth pump sources.